1. Field of the Invention
The present invention relates to a shutter device for an image pickup apparatus, and an image pickup apparatus, such as a digital still camera, including the shutter device and having a function of displaying an image captured by an image pickup device on an image display unit, such as a liquid crystal display panel.
2. Description of the Related Art
In an image pickup apparatus such as a digital still camera which performs image capturing by converting an image signal into an electric signal, an image pickup device, such as a CCD sensor or a CMOS sensor, receives an imaging light beam and outputs an photoelectrically converted signal. The signal is converted into image data, and the data is stored in a recording medium, such as a memory card.
As disclosed in Japanese Patent Laid-Open No. 8-220589, such an image pickup apparatus provides a plurality of image pickup sensitivity choices, such as ISO speed ratings of 100, 200, 400, 800, 1600, and 3200.
As the value becomes larger, the sensitivity becomes higher, and a correctly exposed image can be captured with a small quantity of light. Even at a higher shutter speed, a correctly exposed image can be captured.
For example, for image capturing in an environment with insufficient brightness, e.g., inside of a room, or outside of a room at night, a light quantity is increased by increasing the sensitivity, and a shutter speed is increased correspondingly. Thus, an image without blur can be captured.
However, as the sensitivity is increased, more noise components appear in an image. This may deteriorate the image quality.
In addition, particularly in a lens-interchangeable digital single-lens reflex camera, a focal-plane shutter is arranged near an image pickup device. Static electricity is charged by the operation of the focal-plane shutter, and a noise component is produced when the static electricity is discharged.
As a result, when the digital single-lens reflex camera performs image capturing, the image quality may be deteriorated not only by image capturing with an increased sensitivity, but also by the noise component produced by the operation of the focal-plane shutter.
Here, a configuration of the focal-plane shutter and a mechanism of charging of static electricity by the operation of the focal-plane shutter are described.
FIG. 10 is a front view showing a focal-plane shutter in a before-travel state according to related art. An aperture 501a is an opening which guides an imaging light beam onto a film or an image pickup device. A shutter base plate 501 has a first drive lever shaft 501b and a second drive lever shaft 501c. A first drive lever 502 is rotatably mounted to the first drive lever shaft 501b. A second drive lever 503 is rotatably mounted to the second drive lever shaft 501c. The first drive lever 502 and the second drive lever 503 are provided with torsion springs (not shown), and hence are urged clockwise. The first drive lever 502 transmits a torque to a first blade group 510 (described later) via a first drive pin 502a. The second drive lever 503 transmits a torque to a second blade group 520 (described later) via a second drive pin 503a. The first drive lever 502 and the second drive lever 503 complete their traveling when the first drive pin 502a and the second drive pin 503a contact semicircular rubber pieces 504 and 505.
FIG. 11 is an exploded perspective view showing the focal-plane shutter according to the related art when seen from the obliquely rear side of an opening portion. The second blade group 520, a partition plate 506, a buffer member 531, the first blade group 510, and a cover plate 507 are mounted to the shutter base plate 501 in that order. The partition plate 506 divides a space into a traveling space of the first blade group 510 and a traveling space of the second blade group 520. Also, the partition plate 506 is obliquely arranged with respect to the shutter base plate 501 and the cover plate 507 so as to provide superposing spaces for the first blade group 510 and the second blade group 520. The cover plate 507 has hook insertion holes 507a and 507b which slide along hooks 501h and 501i of the shutter base plate 501, thereby being inhibited into moving in an optical-axis direction. The cover plate 507 is fastened to the shutter base plate 501 by cover plate fastening screws 508 and 509.
FIG. 12 is a rear view showing the focal-plane shutter before traveling according to the related art. FIG. 13 is a rear view showing the focal-plane shutter when traveling of the first blade group 510 is completed according to the related art.
FIG. 14 is a rear view showing the focal-plane shutter when traveling of the second blade group 520 is completed according to the related art. For easier understanding, the partition plate 506 and the cover plate 507 are not illustrated.
The first blade group 510 is rotatably mounted to a first main-arm shaft 501d. The first blade group 510 includes a first main-arm 511, a first sub-arm 512, and four first blades 513, 514, 515, and 516.
The first main-arm 511 is fitted to the first drive pin 502a. The first sub-arm 512 is rotatably mounted to a first sub-arm shaft 501e. The four first blades 513, 514, 515, and 516 are rotatably mounted to the first main-arm 511 and the first sub-arm 512 by blade caulking dowels 517.
Similarly, the second blade group 520 is rotatably mounted to a second main-arm shaft 501f. The second blade group 520 includes a second main-arm 521, a second sub-arm 522, and four second blades 523, 524, 525, and 526.
The second main-arm 521 is fitted to the second drive pin 503a. The second sub-arm 522 is rotatably mounted to a second sub-arm shaft 501g. 
The four second blades 523, 524, 525, and 526 are rotatably mounted to the second main-arm 521 and the second sub-arm 522 by blade caulking dowels 517.
When the first drive lever 502 is rotated by the torsion spring (not shown), the first blade group 510 travels from a deployed state in FIG. 12, in which the aperture 501a is closed, to a superposed state in FIG. 13, in which the aperture 501a is open. The buffer member 531 is made of rubber or the like. When traveling of the first blade group 510 is completed, the first blades 513, 514, 515, and 516 contact the buffer member 531.
When the second drive lever 503 is driven, the second blade group 520 travels from a superposed state in FIG. 13, in which the aperture 501a is open, to a deployed state in FIG. 14, in which the aperture 501a is closed. Hence, the aperture 501a is covered.
FIGS. 15A to 15D are conceptual diagrams showing a mechanism in which static electricity is charged due to the operation of the focal-plane shutter of the related art.
FIG. 15A illustrates a before-travel state of the focal-plane shutter. At this time, the first blade group 510, the partition plate 506, and the cover plate 507 are not moved. Hence, static electricity is not charged.
FIG. 15B illustrates a traveling state of the first blade group 510. At this time, the first blade group 510, the partition plate 506, and the cover plate 507 are charged with static electricity because of friction among the first blade group 510, the partition plate 506, and the cover plate 507.
FIG. 15C illustrates a travel completed state of the first blade group 510. At this time, the first blade group 510 contacts the buffer member 531 (illustrated in FIGS. 12 to 14), and hence the first blade group 510 is largely vibrated in a traveling direction and a direction orthogonal thereto. Owing to this, contact charging and separation charging occur between the first blade group 510 and the partition plate 506, and between the first blade group 510 and the cover plate 507.
In particular, the vibration of the first blade group 510 causes the partition plate 506 to be vibrated, and hence, contact charging and separation charging also occur between the partition plate 506 and the cover plate 507, and between the partition plate 506 and the shutter base plate 501. Accordingly, the first blade group 510, the partition plate 506, and the cover plate 507 are charged with static electricity. Similarly, the second blade group 520 is charged with static electricity because of triboelectric charging occurring among the four blades 523 to 526 of the second blade group 520, between the second blade group 520 and the partition plate 506, and between the second blade group 520 and the shutter base plate 501, in the traveling state and in the travel completed state.
FIG. 15D illustrates a deployed state of the first blade group 510 after traveling is completed. At this time, if the first blade group 510 and any of the partition plate 506, the cover plate 507, and the second blade group 520 are charged with static electricity, a potential difference is produced, and discharging occurs due to the potential difference.
In particular, discharging likely occurs when a distance between the first blade 513, which closes the aperture 501a first among the first blade group 510, and any of the partition plate 506, the cover plate 507, and the second blade group 520 becomes a predetermined distance that causes discharging to occur.
The state shown in FIG. 15D is in the middle of a process in which the image pickup device reads accumulated charges. Hence, a noise component produced by discharging may adversely affect an image.
In light of the circumstances, Japanese Patent Laid-Open No. 2006-114989 discloses that a conductive shutter blade is configured to have a potential at the ground level by using a conductive cover plate which contacts the shutter blade, thereby preventing a focal-plane shutter from being charged.
Also, Japanese Patent Laid-Open No. 2006-227186 discloses that a shutter base plate and a cover plate are formed of a conductive material and electrically connected to the ground.
However, when the shutter blade is configured to have the potential at the ground level by using the cover plate which contacts the shutter blade like the shutter device disclosed in Japanese Patent Laid-Open No. 2006-114989, a partition plate may be vibrated as shown in FIG. 15C. Thus, contact charging and separation charging occur at the shutter base plate, and the partition plate is charged with static electricity.
In addition, as disclosed in Japanese Patent Laid-Open No. 2006-227186, when the shutter base plate and the cover plate are electrically connected to the ground, referring to FIGS. 15B and 15C, triboelectric charging, contact charging, and separation charging occur at the partition plate and the shutter blade, and the partition plate and the shutter blade are charged with static electricity.